The recent trend to portable and wireless computing on the one hand and commodity prices for cell phones and personal computers (PCs) on the other hand has created a need for smaller integrated circuits (IC), IC packages, and electronic products that are low in cost, but capable of high performance. Wafer level packaging (WLP), including flip chip technologies, addresses this need.
WLP is IC packaging formed at the wafer level. With WLP, IC interconnects are fabricated en masse on the wafer, and complete IC modules can be built on the wafer before it is diced. Benefits gained using WLP include, for example, increased I/O density, improved operating speeds, enhanced power density and thermal management, and decreased package size.
One of the keys to WLP is the build up of flip-chip conductive interconnect bumps on the wafer. These interconnect bumps serve as electrical and physical connections of the semiconductor components to a printed wiring board. Several methods of forming interconnect bumps on semiconductor devices have been proposed, for example, solder plate bumping, evaporation bumping, conductive adhesive bonding, stencil printing solder bumping, stud bumping, and ball placement bumping. Of these techniques, it is believed that the most cost effective technique for forming fine pitch arrays is solder plate bumping, which involves a combination of a temporary photoresist plating mask and electroplating. This technique is being rapidly adopted as full-area interconnect bump technology for high value-added assemblies such as microprocessors, digital signal processors, and application specific integrated circuits.
In WLP, front-end IC fabrication and back-end IC assembly are performed at the wafer foundry. Immediately after wafer fabrication, but before testing, connections (e.g., solder bumps) are formed on the wafer. Then, testing and burn-in of the connections is done before singulating into packaged ICs. Flip chip technologies form electrical connections for face-down electrical components on substrates, circuit boards, or carriers using conductive bumps on IC bond pads.
During the WLP process, photolithography steps are required to delineate patterns on the wafers, such as for bond pad distribution and solder bump build-up. The photolithographic process includes stripping the photoresist and removing etch residues. Failure to effectively remove these materials can result in contamination, yield loss, downstream problems in testing and board-level assembly, and reliability fallout in the field.
Photoresists are commonly composed of acrylic resins, polyvinyl cinnamates, diazo compounds, phenol-formaldehydes, or other similar film-forming materials. Photoresists can be applied to the wafer surface dry (e.g., by lamination) or wet (e.g., by spin coating), as illustrated in FIGS. 1 and 2, respectively. Photoresists are further polymerized or cross-linked by ultraviolet light into hard, chemically-resistant films during photolithographic processing.
In the manufacture of semiconductor wafers and printed circuit boards (PCBs), a substrate is coated with photoresist. The photoresist is exposed to actinic radiation and then either the exposed or unexposed photoresist is removed with a suitable developer to produce a pattern in the remaining photoresist. The remaining photoresist protects covered areas of the underlying substrate. Exposed areas are either etched away (e.g., using wet etchants or plasma etching) or have additional materials deposited on them (e.g., via sputtering, chemical vapor deposition, electrochemical plating or electrode-less plating). A semiconductor wafer substrate may include on its surface exposed silicon, oxides, nitrides of silicon, low k dielectrics, or metals (e.g., copper, aluminum, tungsten, nickel, tin, lead, silver, gold, or alloys). A PCB may include many of the same materials. In addition, organic materials such as resists and fluxes are used in WLP for bump formation (e.g., copper post electroplating and solder paste bumping).
After etching or deposition, the remaining photoresist must be removed. The material that remains on the substrate is more difficult to remove. The challenge is to remove only the photoresist and not etch or corrode any other material or leave any residue from either the stripper or the photoresist. It is difficult to find a stripper that is selective in that manner, i.e., that strips or removes photoresist without attacking the other exposed materials in a processed wafer or PCB. What is desired is a stripper that produces an acceptable level of corrosion, below which further processing is unaffected and the electrical operation of the product is unaffected.
Conventional photoresist compositions and processes are not compatible with WLP processing, in part because of the high solder re-flow temperatures (e.g., 150° C.-400° C.) and large resist thicknesses used in WLP processing. Thus, new photoresists for WLP have been developed which, in turn, require new compositions and processes for photoresist stripping and residue removal.
Photoresist stripper products formulated around methyl-2-pyrrolidone (NMP) alone and NMP with alkanolamines, such as PRS100 from Baker, AZ400T from Clariant and EKC830 from EKC, are not effective for WLP because the process conditions effect the photoresist, making it difficult to remove due to, for example, cross linking and encrustation, making stripping of this photoresist unpredictable. Strippers migrating from conventional, low-density PCB processing, such as dimethylsulfoxide (DMSO) with alkaline base (e.g., NaOH or KOH) are not desirable for WLP packaging and high-density PCB applications due to possible metal ion contamination of the IC. Because the wafer level interconnect must be located in the active area of the die, very high input/output (I/O) ICs require very small solder balls with very tight pitches. For example, FIG. 1 shows an array of 30 micron solder balls with 100 micron pitch. High-density PCBs typically require using 25 micron or better photolithography for small solder balls on tight pitches to match the high I/O IC requirements. As the size of the solder balls decrease, the amount of unremoved photoresist required to prevent reliable electrical contact at a solder ball also decreases. Thus, it becomes increasingly important to completely remove the photoresist for smaller solder balls to provide the required quality of performance. Moreover, because the thicker photoresist layer (e.g., 10×) for WLP and PCB applications, compared with at the IC level, it is not predictable to one of skill in the art whether and which strippers used with ICs would work for WLP and PCB applications. Moreover, it is less acceptable to remove the photoresist by a lift-off process rather than a dissolved process due to the possibility of redeposition. Therefore, a need exists for formulations that increase the amount of photoresist dissolved as compared with lift-off.
PCBs are manufactured by plating a thin layer of copper on a substrate (e.g., a glass, ceramic, plastic film or epoxy-glass laminated board). A circuit pattern is formed in the copper layer using photoresist masking and copper etchant solutions. Alternatively, plating copper over a patterned photoresist layer can create the circuit pattern. In either case, the exposed highly crosslinked photoresist must be removed from the PCB substrate. Compositions and methods for removing photoresist from PCBs are described in U.S. Pat. Nos. 3,650,969; 3,600,322; 3,147,224; 3,075,923; 4,269,724; 4,278,577; 3,789,907; 3,625,763; 3,813,309; 3,625,763; 4,483,917; and 4,592,787. Most of the photoresist strippers disclosed contain methylene chloride, which is extremely toxic and is a cancer-causing agent.
Conventional photoresist strippers contain solvents and alkaline bases. Examples of solvent/alkaline mixture types of photoresist strippers that are known for use in stripping applications include dimethylacetamide or dimethylformamide and alkanolamines as described in U.S. Pat. Nos. 4,770,713 and 4,403,029; 2-pyrrolidone, dialkylsulfone and alkanolamines as described in U.S. Pat. Nos. 4,428,871, 4,401,747, and 4,395,479; and 2-pyrrolidone and tetramethylammonium hydroxide as described in U.S. Pat. No. 4,744,834. U.S. Pat. No. 5,962,197 describes a stripper for removing photoresist or solder masks using a mixtures of solvents, surfactants and 0.1 to 5% potassium hydroxide with water contents less than 1%. Potassium hydroxide, for example, causes undesirable oxidation effects on copper substrates, while less than 1% water causes stripped material to be less soluble or insoluble in the stripper. U.S. Pat. No. 5,091,103 describes photoresist stripper compositions comprising N-alkyl-2-pyrrolidone, 1,2-propanediol and tetraalkyammonium hydroxide. U.S. Pat. No. 5,846,695 discloses aqueous solutions of quaternary ammonium hydroxides, including choline, in combination with nucleophilic amines and sugar and/or sugar alcohols, for removal of photoresist and photoresist residues in integrated circuit fabrication. However, this patent requires sugar and/or sugar alcohols to prevent corrosion while the present invention is sugar and/or sugar alcohol free while still providing low rates of corrosion. Unfortunately, these photoresist strippers, as well as other aqueous strippers, are do not completely remove hard baked photoresist and attack the underlying substrate metallurgy, specifically where copper is used as the wiring material.
As the distance between copper lines in PCBs decreases (or, equivalently, as the pitch (lines/distance) in PCBs increases), conventional strippers become ineffective and the amount of photoresist remaining on the substrate increases. Complete resist stripping is needed to control plating distribution, to reduce over plating, and to avoid shorts on costly fine-line PCBs.
A new generation of photoresist stripper compositions and processes are required to address these problems in WLP and PCB manufacturing.